1. Field of the Invention
The present invention relates to an accumulator fuel injection system for injecting a fuel from a common rail, in which a high-pressure fuel supplied under pressure by a fuel supply pump is accumulated under pressure, into each cylinder of an internal combustion engine, such as a diesel engine, through a fuel injection valve.
2. Description of the Related Art
Conventionally, in a widely known accumulator fuel injection system (for example, refer to Patent document 1), a high-pressure fuel is supplied under pressure to a common rail, and is accumulated under pressure therein, by a fuel supply pump (hereinafter referred to as the supply pump) rotatably driven by an internal combustion engine such as a diesel engine (hereinafter referred to as the engine), and, at the same time, the high-pressure fuel accumulated in the common rail under pressure is distributed to two or more electromagnetic fuel injection valves (hereinafter referred to as injectors). Each of the electromagnetic fuel injection valves is mounted on each cylinder of the engine and the high-pressure fuel is supplied by injection to each cylinder of the engine through these injectors.
Here, a fuel path leading from a fuel tank to a pressure chamber of the supply pump is provided with a suction flow control valve. The suction flow control valve is an electromagnetic actuator, which changes the flow rate of the high-pressure fuel to be supplied under pressure to the common rail from the pressure chamber of the supply pump by adjusting the flow rate of fuel sucked into the pressure chamber. The flow rate of the sucked fuel is adjusted by adjusting the period of time during which a valve body is lifted or the opening degree of the valve-port area, in accordance with a current applied to a pump drive circuit.
The suction flow control valve employs a normally-open electromagnetic valve, the opening degree of which reaches a full open state when the supply of current is suspended. Because of this, when an abnormal condition, such as breakage of a cable in a wire harness for transmitting a pump drive signal, occurs, the suction flow control valve abnormally fully opens (this abnormal state is referred to the “full open abnormality” hereinafter), and the supply pump excessively supplies a high-pressure fuel under pressure. Under this condition, the fuel pressure in the high-pressure piping path leading from the pressure chamber of the supply pump to the oil reservoir of the injector of each cylinder via the common rail, especially the fuel pressure in the common rail, may exceed a high limit pressure over which the common rail is not allowed to operate. Therefore, a pressure safety valve (hereinafter referred to as a pressure limiter), which opens when the fuel pressure in the common rail exceeds the high limit pressure, is attached to the common rail in preparation for such an abnormally high pressure and thus the reliability of an accumulator fuel injection system is ensured.
[Patent Document 1]
Japanese Unexamined Patent Publication (Kokai) No. 2001-295685
However, in a conventional accumulator fuel injector system, there may be a case where the pressure limiter does not open even if an abnormally high pressure is produced in the accumulator fuel injector system because of the full open abnormality of the flow control valve. In other words, there may be a case where the common rail pressure remains at a higher pressure (abnormally high-pressure region) than an operating pressure (actually used pressure), even though a pressure limiter (P/L) operating pressure region is not reached, as shown by the change in the fuel pressure in the common rail (hereinafter referred to as the common rail pressure) denoted by the solid line in FIG. 2.
For example, when the flow rate of fuel to be supplied to the common rail under pressure (flow rate of fuel to be discharged by the supply pump) is low and the flow rate of fuel to be injected by and the flow rate of fuel leaking from the injector are high, the common rail pressure is kept in the abnormally high-pressure region. Particularly, when an engine speed (revolution) is low such as when the engine is started (cranked), even if a full open abnormality occurs at the suction flow control valve, the flow rate of fuel to be sucked into the pressure chamber of the supply pump does not increase beyond a limit determined by the engine speed. Because of this, there is the possibility that the common rail pressure is kept in the abnormally high-pressure region.
Under this condition, the pressure in the high-pressure piping path is kept high, even though an allowable pressure of the supply pump and the injector is not exceeded. Because of this, there is the possibility of performance degradation of the supply pump and the injector and occurrence of malfunctions, such as a fuel leakage at piping connecting parts or sealing parts in the high-pressure side fuel path. This causes a problem in that the reliability of the accumulator fuel injection system is degraded.